A color picture tube equipped with in-line electron guns emitting three electron beams onto the same plane is required to have a convergence adjuster which causes the electron beams to converge on the viewing screen.
A color picture tube device of this kind is schematically shown in FIG. 1. This device comprises a glass bulb 501 including a funnel 502 and a neck 503. The tube further includes a fluorescent screen 504 forming a viewing screen, a shadow mask 505, a magnetic shield 506, a deflection yoke 507, purity magnetic rings 508, convergence-adjusting magnets 509 for the central beam 1, other convergence-adjusting magnets 510 for the outer beams 2, and electron guns 511.
In this color picture tube device, static convergence is achieved by causing the two outer beams 2 to converge and then bending the outer beams so as to converge with the central beam.
Since the structures and the functions of the deflection yoke 507, the purity magnetic rings 508, the convergence-adjusting magnets 509 for the central beam, etc. are known, they are not described in detail herein.
FIG. 2 is a view for illustrating a method of achieving static convergence by bringing the outer beams substantially into the center of the viewing screen. The figure shows the central beam 1, the outer beams 2, magnetic lines of force 5, and electromagnets or permanent magnets 24.
The orbits of the outer beams 2 are bent by the Lorentz force of magnetic lines of force formed by the quadrupole field, which is created by the magnets 24 of the same magnitude. Thus, the two outer beams 2 converge on the viewing screen.
FIGS. 3a and 3b illustrate ring magnets which together act as a static convergence adjuster for the outer beams and produce the aforementioned quadrupole field. FIG. 3(a) shows the first ring magnet, and FIG. 3(b) shows the second ring magnet. These two magnets are held by magnet holders 23-a and 23-b, respectively, which have operation protrusions 23-1 and 23-2, respectively.
Each ring magnet has north poles and south poles which are spaced 90.degree. from each other circumferentially of the magnet holder 23-a or 23-b. Like poles are positioned diametrically opposite to each other. When the operation protrusions 23-1 and 23-2 of the ring magnets of FIGS. 3(a) and 3(b) are superimposed, the north poles of one ring magnet are placed on the south poles of the other, so that the magnetic lines of force are canceled out. Under this condition, the ring magnets are installed on the neck of the color picture tube. The operation protrusions 23-1 and 23-2 are rotated in opposite directions with the magnet holders. In this way, the electron beams are subjected to the quadrupole field. The outer beams 2 are made to converge by adjusting the intensity of the magnetic field. The intensity of the field is maximized when the two ring magnets of FIGS. 3(a) and 3(b) are rotated through 45.degree. in opposite directions. The quadrupole field produced by the ring magnets for achieving the convergence of the outer beams is characterized in that it is maintained in a square arrangement.
A color picture tube having no magnetic field for convergence adjustment is generally designed so that a so-called under convergence is achieved, i.e., the arrangement of the electron beams at the position of the neck of the picture tube is the same as the arrangement of the beams on the viewing screen. The quadrupole field is so set up that when the convergence of the outer beams is adjusted, the outer beams are deflected towards the central beam. A device of this kind is disclosed in, for example, Japanese Patent Laid-Open No. 200648/1986.
In the conventional technology device, the quadrupole field takes a square arrangement as shown in FIG. 2. The pattern of the magnetic lines of force is of the pincushion type.
FIG. 4 illustrates the effect of the quadrupole field upon the outer beams. The magnetic line of force 5 passing through each outer beam 2 is curved, i.e., the pattern of the magnetic lines of force is of the pincushion type. When forces 7 and 8 directed toward the central beam, i.e., horizontally, act on the beam 2, forces 9 which are perpendicular to the forces 7 and 8 and are directed vertically act on the beam 2. As a result, the outer beam 2 is bent toward the central beam, i.e., directed horizontally, and, at the same time, diverge vertically. This deteriorates the vertical resolution.